This invention is related to positioning devices in microelectromechanical systems (MEMS) and, more particularly, to using a bimorph actuator that has been modified to affect the power off position of the positioning device.
MicroElectroMechanical (xe2x80x9cMEMxe2x80x9d) devices comprise integrated micromechanical and microelectronic devices. The term xe2x80x9cmicrocomponentxe2x80x9d will be used herein generically to encompass microelectronic components, micromechanical components, as well as MEMs components. The advances in microcomponent technology have resulted in an increasing number of microcomponent applications. Accordingly, a need often arises for precise positioning of microcomponent devices. For example, it is often desirable to position a microcomponent in alignment with a target position. For instance, for certain applications it may be desirable to align a microcomponent with another device. Because of the small size of microcomponents, they often require very precise positioning (e.g., precise alignment with another device). For example, in some cases a misalignment of only a few microns may be unacceptable. In fact, in some cases the size of the microcomponent being aligned may be only a few microns. Also, microcomponents present particular difficulty in handling and positioning operations.
Plastic deformation of single composition MEM devices is known. For example, U.S. Pat. No. 6,261,494, entitled METHOD OF FORMING PLASTICALLY DEFORMABLE MICROSTRUCTURES, the disclosure of which is incorporated herein by reference herein, teaches a method of plastically deforming MEM structures. Copending U.S. patent application Ser. No. 09/932,489, filed Aug. 17, 2001, and entitled SYSTEM AND METHOD FOR PRECISE POSITIONING OF MICROCOMPONENTS, the disclosure of which is hereby incorporated by reference herein, teaches deforming microactuators to fix the position of microcomponents.
Stress relaxation in bimorph microstructures is also known. For example, in the article entitled xe2x80x9cStress Relaxation of Gold/Polysilicon Layered MEMS Microstructures Subjected to Thermal Loading,xe2x80x9d by Zhang and Dunn, stress relaxation is studied for gold within a gold/polysilicon bimorph device. Creep may occur at elevated temperatures and may cause deformation if the bimorph is exposed to an elevated temperature over a period of time. Accordingly, yielding mechanisms in some materials are time and temperature dependent as well as stress dependent.
Microcomponents are commonly implemented in the field of optoelectronics. Generally, when coupling optoelectronic components, alignment is very important. That is, alignment of optoelectronic components is often critical for proper operation of an optoelectronic device. A relatively slight misalignment of optical components may drastically alter an optical device""s performance. For example, accurate alignment of components is often important for ensuring proper propagation of an optical signal to/from/within an optoelectronic device. For instance, optoelectronic modules, such as optoelectronic receivers and optoelectronic transmitters commonly require proper alignment of microcomponents therein for proper operation. In general, proper alignment is desired to minimize the amount of attenuation within such optoelectronic devices.
One microcomponent that often requires proper alignment is an optical fiber. For example, in an optoelectronic receiver, a fiber is aligned with an optical detector, typically a PIN photodiode. Very large fibers may have light-guiding cores with a diameter of approximately 1 millimeter (mm) or 1000 microns (xcexcm), but such fibers are rarely used in communications. Standard glass communication fibers have cladding diameter of 125 xcexcm and light-guiding cores with diameter of approximately 8 to 62.6 xcexcm. Proper alignment of the end of the optical fiber (which may be referred to as the xe2x80x9cfiber pigtailxe2x80x9d) with the optical detector is important to ensure that a light signal is properly received by the optical detector. Similarly, in an optoelectronic transmitter, an optical fiber is aligned with a light source, such as a light-emitting diode (LED) or laser diode. Proper alignment of the end of the optical fiber with the light source is important to ensure that a light signal is properly communicated from the light source to the optical fiber.
The difficulty in achieving proper alignment of optical fiber is often increased because of variances in the size of fiber core diameters. For example, typical commercial graded-index fiber commonly specify a 50 xcexcm nominal fiber core diameter that may vary within a tolerance of xc2x13 xcexcm. Also, alignment/positioning of the light-guiding core within the sleeve of a fiber optic cable often varies (i.e., the core is not always centered within the sleeve), thereby further increasing the difficulty of properly designing a fiber with another optoelectronic device.
Various techniques have been developed for handling and positioning microcomponents, such as optical fibers. According to one technique, a high-precision, external robot is utilized to align microcomponents within devices. However, such external robots are generally very expensive. Additionally, external robots typically perform microcomponent alignment in a serial manner, thereby increasing the amount of time required for manufacturing microcomponent devices. That is, such robots typically perform alignment for one component at a time, thereby requiring a serial process for assembling microcomponents utilizing such a robot.
According to another technique, microactuators, such as electrothermal actuators, may be utilized to align microcomponents, such as optical fibers. For example, microactuators may be integrated within a device to align microcomponents within the device. Accordingly, use of such microactuators may avoid the cost of the above-described external robot. Also, if implemented within a device, the microactuators may enable parallel alignment of microcomponents. That is, multiple devices may have alignment operations performed by their respective microactuators in parallel, which may reduce the amount of time required in manufacturing the devices. Examples of techniques using microactuators integrated within a device to perform alignment of an optical fiber are disclosed in U.S. Pat. Nos. 6,164,837 and 5,602,955, the disclosures of which are hereby incorporated by reference herein.
Once a desired position is obtained for a microcomponent (e.g., alignment with another device) using either of the above techniques, such microcomponent may have its position fixed in some manner such that it maintains the desired position. Various techniques have been developed for fixing the position of microcomponents. According to one technique, an epoxy may be used to fix the position of a microcomponent. In another technique a low melting point bonding material, such as solder, may be used to fix the position of a microcomponent. Exemplary techniques that use solder to fix the position of an optical fiber are disclosed in U.S. Pat. No. 6,164,837, U.S. Pat. No. 5,692,086, and U.S. Pat. No. 5,745,624, the disclosures of which are hereby incorporated by reference herein.
According to another technique, an xe2x80x9cactivexe2x80x9d alignment device may be utilized to fix the position of a microcomponent. Such an alignment device is xe2x80x9cactivexe2x80x9d in the sense that electrical power has to be maintained in order to fix the alignment of a microcomponent. For example, in certain implementations that use microactuators integrated within a device to perform alignment of microcomponents, power to such microactuators must be maintained in order to maintain (or fix) the position of the microcomponents being aligned.
Plastic deformation micro-assembly has been demonstrated using Plastic Deformation Magnetic Assembly (PDMA), such as in the article entitled xe2x80x9cPlastic Deformation Magnetic Assembly (PDMA) of 3D Microstructures: Technology Development and Application,xe2x80x9d by J. Zou, J. Chen and C Liu. However, PDMA technology can only be deformed to one position (i.e. unidirectional) and cannot be adjusted after the assembly step. Also, it requires the use of an external magnetic field and magnetic materials in the actuator itself.
The present invention is directed to a system and method in which a bimorph is used in a microelectromechanical actuator to modify the power off positioning of the actuator. The bimorph is comprised of two materials that are bonded together, each material having a different thermal expansion coefficient so that they expand at different rates when the temperature is increased. The temperature may be increased by applying a current to the bimorph or by applying an external heat source, such as a laser, to the bimorph.
As the bimorph is heated, the materials expand at different rates, thereby causing the bimorph to bend. Normally, when the heat source is removed, the bimorph will return to its original state. However, when the bimorph is heated such that the stress in one of the materials increases beyond the yield point, the bimorph is plastically deformed so that it does not return to its initial state after the heat source is removed. Instead, because of residual stress that is present in the material that was plastically deformed, when the heat source is removed, the bimorph is forced to bend in a direction opposite to the bending due to expansion. Typically, the material that has a higher thermal expansion coefficient will reach its yield point first and, therefore, will be plastically deformed.
As a result of the residual stress in the plastically deformed material, the bimorph assumes a new initial state. When the bimorph forms part of a microelectromechanical actuator, the microactuator assumes a new initial or power off state. Additionally, although the power off state of the microactuator may be modified from an initial position using the method of the present invention, the microactuator can still be moved by heating and thermal expansion.
The present invention has application in many areas, such as the telecommunications and fiber areas discussed above. Other applications, such as in microwave circuits, are also possible. For example, the deformable structures disclosed herein may be used to place inductors at a 90 degree angle to the substrate. In other applications, the invention can be used to adjust the positioning of capacitive plates.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.